All you need to fit the longer belly pan is a couple of extra small brackets off the footrest hanger.(number 24 in the pic)All the other fixings are the same.
I made my own out of a bit of scrap alloy bar,...but you can get them from somewhere like Ultimate Parts for about £13.
The carbon factory belly pans do come up on ebay from time to time,but they aint cheap.I was lucky to get one,... new,old stock,...for £250.Good luck with the search.